Cellulose-enabled orientationally ordered flexible gels

ABSTRACT

Disclosed are cellulose-based flexible gels containing cellulose nanorods, ribbons, fibers, and the like, and cellulose-enabled inorganic or polymeric composites, wherein the gels have tunable optical, heat transfer, and stiffness properties. The disclosed gels are in the form of hydrogels, organogels, liquid-crystal (LC) gels, and aerogels, depending on the solvents in the gels.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a divisional of U.S. Non-Provisional applicationSer. No. 15/868,714, filed Jan. 11, 2018, and entitled“CELLULOSE-ENABLED ORIENTATIONALLY ORDERED FLEXIBLE GELS,” which claimsthe benefit of U.S. Provisional Application No. 62/444,896, filed Jan.11, 2017, and entitled “CELLULOSE-ENABLED ORIENTATIONALLY ORDEREDFLEXIBLE GELS,” the contents of which are incorporated herein byreference to the extent such contents do not conflict with the presentdisclosure.

FEDERALLY SPONSORED RESEARCH

This discovery was made with Government support under grant DMR-1410735awarded by the U.S. National Science Foundation. The Government hascertain rights in the invention.

FIELD

Disclosed are cellulose-based flexible gels containing cellulosenanorods, ribbons, fibers, and the like, and cellulose-enabled inorganicor polymeric composites, wherein the gels have tunable optical, heattransfer, and stiffness properties. The disclosed gels are in the formof hydrogels, organogels, liquid-crystal (LC) gels, and aerogels.Further disclosed are highly transparent and flexible cellulosenanofiber-polysiloxane composite aerogels featuring enhanced mechanicalrobustness, tunable optical anisotropy, and low thermal conductivity.

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1 shows the schematic of fabrication procedures ofcellulose-enabled ordered gels (a) cellulose nanomaterials aqueousdispersion, (b) hydrogel, (c) organogel (d) aerogel, and (e) liquidcrystal gel.

FIG. 2 shows transmission electron microscopy (TEM) images of cellulosenanorods, nanofibers, nanoribbons from different sources: (a) cellulosenanorods from cotton, (b) cellulose nanowires from cotton, (c) cellulosenanowires from wood pulp, and (d) cellulose nanoribbons from bacterialcellulose.

FIG. 3 shows photographs of nematic cellulose (a) hydrogel, (b)organogel, (c) aerogel, (d) nematogel, and (e) scanning electronmicroscopy (SEM) image of the ordered aerogel. The scale bar is 1 cm.

FIG. 4 shows photographs of cholesteric (a) cellulose-silicacomposition, (b) silica aerogel, (c) silica nematogel, and (d) SEM imageof the ordered silica aerogel. The scale bar is 5 mm.

FIG. 5 is a schematic representation of a spectrometer assembly that canbe used for haze measurements at diffuse illumination and unidirectionalviewing.

FIG. 6 describes a general schematic of one embodiment for thefabrication of the disclosed aerogels.

FIG. 7 shows the optical pathway of a typical pump-and-probe measurementsystem.

FIG. 8 discloses the components of a hot box apparatus: a metering box(simulating interior temperature) on one side of the window specimen; acontrolled guard box surrounding the metering box; a climate chamber box(simulating exterior temperature) on the other side; and the specimenframe providing specimen support & insulation.

FIGS. 9A-9C various Tempo-oxidized cellulose nanoparticle modifyingagents.

FIG. 9A depicts surface modification by allylamine. FIG. 9B depictssurface modification by a 2-(carbamoyloxy)-N,N,N-trimethylethanaminiumadduct. FIG. 9C depicts surface modification by example a methoxypolyethylene glycol amine (mPEG-amine).

FIG. 10 depicts the disclosed process for formingpolymethylsilsesquioxane (PMSG) network cellulosic hydrogels, organogelsand aerogels process in general.

FIG. 11 is a photograph showing the optical transparency of a hydrogelformed from the disclosed process.

FIG. 12 is a photograph showing the optical transparency of an organogelformed from the disclosed process.

FIG. 13 is photograph showing the optical transparency of an aerogelformed from the disclosed process wherein the surface modifying agent isallylamine.

FIG. 14 is a photograph of an aerogel formed by the disclosed processwherein the surface modifying agent is an m-PEG-amine having an averagemolecular weight of 5000 daltons.

FIG. 15 is a photograph of an aerogel formed by the disclosed processwherein the surface modifying agent is carbamoylcholine chloride.

FIG. 16 depicts that the carbamoylcholine chloride-capped TOCN-PMSQaerogels exhibit hydrophobic surface characteristics.

FIGS. 17A-17C are transmission electron microscopy (TEM) micrographs ofthe disclosed aerogels at various magnifications. FIG. 17A shows thatthe colloidal dispersions consist of mostly individualized TOCNs, eachof diameter D_(c)≈5 nm and length L_(c)≈1-2 μm. FIGS. 17B and 17C arescanning electron microscopy (SEM) that depict the well-defined anduniform-diameter 10-15 nm nanofibers that are formed by polysiloxane.

FIG. 18 shows the visible transmission of a disclosed aerogel.

FIG. 19 shows the haze coefficient of a disclosed aerogel.

FIG. 20 depicts the that the PMSQ matrix causes TOCN-PMSQ aerogels toexhibit strong absorption at a wavelength of 6-20 μm that is mainly dueto the Si—O bond.

FIG. 21 shows the measured thermal conductivity of an TOCN-PMSQ aerogelversus sample porosity.

FIG. 22 depicts the comparison of thermal conductivity between anaerogel formed from carbamoylcholine chloride modified nanocellulose(quaternary-amine) and an allylamine modified aerogel.

FIG. 23 depicts the compression stress-strain relation for a TOCN-PMSQaerogel with 0.06 wt. % of TOCN

DETAILED DESCRIPTION

As used herein, a “gel” is understood to be a substantially dilutecross-linked system that exhibits no flow when in the steady state. Theprimary constituent of the gel is the ambient fluid surrounding it,whose form can be a liquid or gas. Prefixes such as “aero,” “organo,”“hydro,” and variations are understood to indicate the ambient fluid inthe cross-linked gel matrix and primary component of the gel material.

The disclosed gels contain cellulosic nanocomposites that are aligned inordered liquid crystal phases. As such, the disclosed gels allow theformulator to adjust the optical transmissivity of the gel, therebyconfiguring the optical properties of the gel to range from opaque totransparent. In addition, the properties can be adjusted to inter actwith a wide range of the electromagnetic spectra, for example, from thevisible spectrum to infrared spectrum. In one embodiment, the thermalconductivity of the gel can be adjusted. The bulk properties of thedisclosed gels, for example the level of stiffness or flexibility can beadjusted by the choice of the constituent cellulosic material, forexample, nanorods, ribbons, fibers, and the like, as well as, theconcentration of these materials in the resulting gels.

As used herein, a “film” and variations indicate non-porous lamellaeranging in thickness from about 10 nm to 1 mm and arbitrary lateralextent.

As used herein the term “cross-section” means width and the terms areused interchangeably. The disclosed cellulosic nanomaterials have awidth from about 10 nm to about 500 nm. The length of the nanomaterialsis at least ten times the width.

The term “composition” as used herein refers to the disclosed cellulosenanomaterial aqueous dispersions, hydrogels, organogels, aerogels, andliquid crystal gels. The compositions can be a single layer of materialcomprising nanomaterials or the composition can be formed from two ormore distinct layers wherein each layer consists of only one material.As a non-limiting example, one layer can consist of an ordered nematiccellulosic gel onto which a second layer of cholesterically alignedcellulose film is applied thereto. This layering thereby forms a unifiedcomposite material with distinct layers.

The term “hydrogel” as used herein represents a network of cellulosicmaterial as a colloidal gel dispersed in a carrier. In one embodimentthe carrier is water. In another embodiment the carrier is a mixture ofa water compatible (miscible) organic solvent. The cellulosic materialcan be crosslinked or non-crosslinked.

The term “organogel” as used herein is a gel wherein the aqueous phaseof a precursor hydrogel has had substantially all of the water removedand replaced by a water compatible solvent. Non-limiting examples ofcompatible solvents include methanol, ethanol, propanol and isopropanol.In one embodiment, the disclosed organogels have a two dimensionalcross-linked network. In another embodiment the disclosed organogelshave a three dimensional crosslinked network.

The term “aerogel” as used herein refers to a gel derived from thefurther processing of a disclosed organogel as described herein.

The term “liquid crystal gel” refers to the compositions derived fromthe further processing of the disclosed organogels as described herein.

The term “nanomaterial” refers to the disclosed cellulosic material. Thewidth of these materials is in the nanometer range, whereas the lengthof the cellulosic material can vary from nanometer length to micrometer.The terms “nanomaterial,” “cellulosic material” and “cellulosicnanomaterial” are used interchangeably throughout the presentdisclosure.

The term “nematic” as used herein refers to a composition wherein thedisclosed cellulosic materials are aligned in one direction. Inaddition, the cellulosic materials are free to flow and their center ofmass positions are randomly distributed, but still maintain theirlong-range directional order. The disclosed nematic compositions areuniaxial: they have one axis that is longer and preferred, with theother two being equivalent.

The term “cholesteric” as used herein refers to a composition with ahelical structure and which is therefore chiral. The disclosedcholesteric compositions are organized in layers with no positionalordering within layers, but a director axis which varies with layers.The variation of the director axis can be periodic in nature. Ifpresent, the period of this variation (the distance over which a fullrotation of 360° is completed) is known as the pitch, which can beadjusted by the formulator, and the degree of pitch determines thewavelength of electromagnetic radiation which is reflected.

The abbreviation “TOCN” as used throughout the specification means“TEMPO-oxidized cellulose nanofibers.”

The term “low molecular weight compounds comprising a cationic moiety”means a compound that has a moiety that can react with an oxidizedcellulose carboxyl group in addition to a separate cationic moiety.Non-limiting examples of units that can react with an oxidized cellulosecarboxyl group include hydroxyl group, an amino group, a thiol group,and the like. A non-limiting example of a cationic moiety includes aquaternary ammonium group.

In one aspect of the present disclosure are compositions comprisingcellulosic nanoribbons that are aligned together and which orientationcan be adjusted by the formulator. The disclosed nanoribbons have aspectratios from about 1:100 to about 1:1000. In one aspect, the disclosednanoribbons can be used to form a nematic flexible gel.

These cellulose-based flexible gels, can comprise cellulose ribbons,fibers, and other constituent-particle structures having in oneembodiment aspect ratios of about 1:1000. These flexible gels are formedfrom linking the cellulose particle networks within the material. Theoriginal cellulose solvent which is used for the formation of the gelnetwork can be retained or replaced to yield a variety of gel types, forexample, hydrogels, alcogels, aerogels, and liquid-crystal gels. The useof the disclosed cellulosic material to form the gel network allows theformulator to adjust the flexibility of the gels.

For example, cellulosic nanomaterials having a larger aspect ratioresults in less elastomeric crosslinking. The aspect ratios allow for agreater degree of flexibility or rigidity depending upon the selectionof cellulosic nanomaterial. In addition, the cellulose particles can beordered through chemical and mechanical means to yield a lyotropicliquid crystalline dispersion with ordered phases. The ordering can bepreserved during the cross-linking process to form various ordered gels,non-limiting examples of which are given above. FIG. 1 illustrates thefabrication procedure of cellulosic ordered gels.

In addition to flexibility, the optical transmissivity of the disclosedgels can be adjusted to range from opaque to transparent. The degree ofopaqueness or transparency can also be matched of any wavelength orrange of wavelength in the electromagnetic spectrum. These results canbe obtained by adjusting the various properties of the disclosedcomposites, i.e., density of cellulosic nanomaterial or sizedistribution. Also, the addition of adjunct ingredients such as liquidcrystal materials can be used to tune the optical properties of thedisclosed composites.

A further property which can be tailored to the needs of the formulatoris the degree of thermal resistance displayed by the gels. Severalfactors enable the adjustment of the thermal resistive properties: (1)the intrinsically low thermal conductivity of cellulose, (2) therarefication of fluid within the cellulose network thereby regulatingthe thermal convection, and (3) the thermal conductivity and convectionproperties of the fluids which comprise the cellulose-gel network.

In another aspect of the present disclosure are compositions comprisingcellulosic nanorods that are aligned and which orientation can beadjusted by the formulator. The disclosed nanoribbons have aspect ratiosfrom about 1:10 to about 1:100. In one aspect, the disclosed nanorodscan be used to form compositions with a cholesteric phase.

In one embodiment the disclosed nanocrystals form ordered films that canbe ordered into a cholesteric phase in the film to form a periodicstructure whose pitch and pitch gradient are adjustable for broad-bandBragg reflection of incident electromagnetic radiation. In anotherembodiment the resulting ordered gels are obtained because of the smallrelative aspect ratios of the cellulose nanorods or similarnanomaterials that comprise the nanocrystals. Nanorods result in theformation of different phases than other nanomaterials, i.e.,nanofibers. Because of this fact broad-band reflection is enabled inordered cellulose gels that are formed from cellulose structures withaspect ratios of about 1:10 to about 1:100.

As such, the mechanical flexibility, optical transmissivity, and thermalresistance can be configured by tuning the same parameters describednanofibers, except that those parameters now refer specifically tocellulose nanorods or other geometrically anisotropic cellulosestructures.

A further aspect of the present disclosure relates to compositestructures comprising lamellae that are formed from the disclosedaerogels or liquid crystal gels. Composite structures with lamellae canbe formed from the disclosed compositions that comprise nanofiber-likecellulosic materials (nematic phase) or from nanorod-like cellulosicmaterials (cholesteric phase). These composite structures comprise aplurality of layers.

In a still further aspect of the present disclosure are compositestructures wherein an amount of the cellulosic nanomaterial is replacedwith one or more adjunct materials which can affect the alignment of thecomposite nanomaterials. In one embodiment a portion of the cellulosicnanomaterial is replaced with liquid crystals. As such, the nematicphase or cholesteric phase gels can have the liquid phase substituted byother anisotropic organic or inorganic materials. In one embodimentsilica is introduced into the hydrogel. In another embodiment liquidcrystal material, for example,1-(trans-4-hexylcyclohexyl)-4-isothiocyanatobenzene can replace thecarrier of the hydrogel. In another embodiment, after the incorporationand alignment of non-cellulosic materials, the cellulose can bepartially or totally removed, by chemical means, to yield gels and filmswith partial or total cellulose substitution.

In a yet further aspect of the present disclosure are lamellae whichcomprise the disclosed composite structures. According to this aspectthe lamellae are formed from two or more distinct layers wherein eachlayer comprises different materials. The cellulosic nanomaterials thatcomprise each layer can further serve as a template and can besubstituted partially or totally by other materials such as silica orother polymers.

Further disclosed are methods for the preparation of the compositestructures.

Cellulose Nanomaterials

The disclosed cellulose nanomaterials can have a variety of shapes andcross-sectional geometries that depend upon the nanomaterial's naturalsource and the process used to produce the particles. A disclosedcellulose nanomaterial can have a shape that is a rod, fiber, ribbon,whisker, and the like.

The disclosed cellulose nanomaterials can be obtained by chemical ormechanical treatment of a variety of natural sources, for example,cotton, soft wood pulp, hard wood pulp, tunicate and bacterial celluloseand the like. Typically nanorods and nanofibers can be obtained frommultiple sources, for example, cotton and bacteria.

In one embodiment, the disclosed nanocellulose can have a length fromabout 10 μm to 100 μm with cross sections from about 10 nm to 50 nm. Inanother embodiment, the disclosed nanocellulose can have a length fromabout 1 μm to 10 μm with cross sections from about 3 nm to 10 nm. In ayet another embodiment, the disclosed nanocellulose can have a lengthfrom about 100 nm to 1 μm with cross sections from about 3 nm to 10 nm.FIG. 2 shows TEM micrographs of cellulose particles with characteristiclength scales of the aforementioned embodiments: (a) cellulose nanorodsof 10 nm×200 nm depends upon the chemical treatment from cotton, (b)cellulose nanowires of 7 nm×800 nm from cotton, (c) cellulose nanowiresof 4.8 nm×1 μm from wood pulp, and (d) cellulose nanoribbons of 10 nm×50nm×10 μm from bacterial cellulose.

The cellulose nanomaterials can be obtained by chemical hydrolysis ofnatural cellulose by sulfuric acid, hydrochloric acid, etc. or by2,2,6,6-tetramethylpiperidine-1-oxyl radical (TEMPO)-mediated oxidation.

Alignment Methods

The cellulose nanomaterials can be aligned by linear or circularshearing for nematic or cholesteric ordering, respectively. Nematicordering of the disclosed cellulose nanofibers is shown in FIG. 1(b). Inone embodiment, the cellulose nanomaterial dispersion can be confinedbetween glass plates in a mold such that, when a shear stress is appliedfrom the plates in the specified direction, individual nanocelluloseparticles align to form a singular director alignment across theconfined dispersion. In another embodiment, the nanocellulose suffersunidirectional alignment under extrusion from a sufficiently smalldiameter nozzle, syringe, or similar device. With extrusion alignment,no confining plates are needed such that the aligned dispersion takesthe form of a narrow bead, with linear extent much greater than itscross-sectional extent, which rests on a supportive substrate or otherstructure. In yet another embodiment, the helical axis of cellulosenanorods in cholesteric phase can be aligned by anticlockwise circularshearing during the evaporation of the dispersion.

The disclosed cellulose nanomaterials can also be aligned by magnetic orelectric fields. In one embodiment, the magnetic anisotropy of cellulosenanomaterial's relative permeability can be exploited to cause uniformalignment of nanocrystals. Under sufficiently strong magnetic fields(about 1 T), uniform alignment of nanocrystals perpendicular to themagnetic field is achieved through the magnetic interaction of theinduced magnetic dipole moments of the cellulose nanomaterial with theapplied magnetic field. In another embodiment, an oscillating electricfield can also be used to aligned the cellulose nanomaterials in asimilar manner.

Ordered Hydrogels

The alignment of the disclosed cellulose nanomaterial dispersions can bepreserved by its conversion to a hydrogel, example embodiments which areshown in FIG. 1(b) and FIG. 3(a). The extent of cross-linking of thecellulose nanomaterial establishes the degree to which uniform orderingis preserved in the dispersion. For example, a low level of crosslinkingprovides a weaker gelation. Conversely a greater degree of cross-linkingyields firmer gelation. Gelation is accomplished by the addition of anacid, photoacid generator and exposure to light, alcohol, or othercationic exchange reagent to a uniformly ordered cellulose nanomaterialdispersion. In one embodiment, hydrochloric, acetic, nitric, sulfuric,and phosphoric acids can be added to instigate gelation. In anotherembodiment, Ca²⁺ can be added to provide cross-linking.

Ordered Organogels

The disclosed ordered hydrogels can be transformed into organogels, asshown in FIG. 1(c) and FIG. 3(b) using a solvent exchange procedure. Forexample, a hydrogel can be gently shaken while immersed in anethanol-filled bath followed by replacing the solvent at regularintervals. In this way, the water is sequentially removed from thematrix and replaced with ethanol. Other organic solvents can substitutefor ethanol. Non-limiting examples of other solvents include methanol,ethanol, isopropanol, butanol, hexane, acetone, dichloromethane,dimethylformamide (DMF), dimethylsulfoxide (DMSO), and toluene.

Ordered Aerogels or Films

The disclosed cellulose aerogels or films herein can be produced fromthe disclosed nano-structured organogel herein above. Example aerogelsare shown in FIG. 1(d) and FIG. 3(c). The resulting cellulosenanomaterial aerogel is porous having a skeleton of about 0.1-99.9%. 1-3(10)% cellulose nanocrystals and a porosity of from about 0.01 to about99.99%. In one embodiment the porosity is from about 97% to about 99%.

Aerogels comprising a low percentage of nanocellulosic material resultsaerogels having a high degree of transparency, for example, from about1% to about 50% by weight of the aerogel. Aerogels comprising from about50% to about 90% by weight of nanocellulosic material results aerogelshaving a high degree translucent scattering.

To prevent deformation and crumbling of aerogels during the drying dueto surface tension and capillary pressure in the ambient atmosphere,supercritical drying, freeze drying, or ambient drying with a lowsurface-tension solvent are used to remove liquid solvent from cellulosenanocrystal composites while maintaining the disclosed liquidcrystalline structure, such as nematic or cholesteric liquid crystallineordering.

As disclosed previously herein, the resulting monolithic cellulosenanomaterial film is a solid material with 100% cellulose composition.Ambient drying is used to remove liquid solvent.

Ordered Liquid-Crystal Gels

The disclosed cellulose LC gels herein can be produced from thedisclosed nano-structured organogel. The organic solvent is replacedwith LC by solvent exchange. For the case of LC gels, the LC functionsas the gel's solvent. FIG. 1(e) portrays a schematic representation ofLC gels while FIG. 3(d) portrays an LC gel whose LC solvent is in thenematic phase. The disclosed compositions can comprise any LC that willserve as solvents for the gels. Non-limiting examples of nematic LCsinclude: 1-(trans-4-hexylcyclohexyl)-4-isothiocyanatobenzene;4′-(hexyloxy)-4-biphenylcarbonitrile;4′-(octyloxy)-4-biphenyl-carbonitrile;4′-(pentyloxy)-4-biphenylcarbonitrile; 4′-heptyl-4-biphenylcarbonitrile;4′-hexyl-4-biphenylcarbonitrile; 4′-octyl-4-biphenylcarbonitrile;4′-pentyl-4-biphenylcarbonitrile; 4,4′-azoxyanisole;4-isothiocyanatophenyl 4-pentylbicyclo[2.2.2]octane-1-carboxylate;4-(trans-4-pentylcyclohexyl)benzonitrile; 4-methoxycinnamic acid;N-(4-ethoxybenzylidene)-4-butylaniline; andN-(4-methoxybenzylidene)-4-butylaniline.

Cellulose-Templated Ordered Inorganic Gels or Films

Inorganic nanomaterials can be incorporated into the heretoforedisclosed ordered gels or films using cellulose nanomaterials as atemplate. As a non-limiting example, FIG. 4 demonstratescellulose-enabled cholesteric (a) cellulose-silica film composition, (b)silica aerogel, and (c) silica LC gel whose solvent is LC in its nematicphase. An SEM image of the ordered silica aerogel is displayed in FIG.4(d). At the final processing stage, the cellulose nanomaterials can beremoved to obtain inorganic gels or films. Alternatively,inorganic/cellulose composite gels or films are formed without etchingthe cellulose nanomaterials. As one non-limiting example, the inorganicgels or films are made of silica, organo-silica, titanium dioxide,aluminum oxide, rare earth oxides, etc. The weight concentration ofinorganic nanomaterial in the inorganic/cellulose composites can rangefrom about 1% to about 99%.

Cellulose-Templated Ordered Polymeric Gels or Films

Polymers can be incorporated into the heretofore disclosed ordered gelsor films using cellulose nanomaterials as a template. At the finalprocessing stage, the cellulose nanomaterials can be removed to obtainpolymeric gels or films. Alternatively, polymeric/cellulose compositesgels or films are formed without etching the cellulose nanomaterials. Asone non-limiting example, the polymeric gels or films are made ofphenol-formaldehyde, melamine-formaldehyde, urea-formaldehyde,poly(acrylic acid), polyester, etc. The weight concentration ofpolymeric nanomaterial in the polymeric/cellulose composites can rangefrom about 1% to 99%.

Surface Functionalization Ordered Gels

The surface properties of the disclosed gels can be modified byfunctionalizing the surface of the cellulosic network. The disclosedaerogels having no surface functionalization are hydrophobic and candissolve upon contact with water. Surface modifiers, example,dimethyloctadecyl [3-(trimethoxysilyl)propyl]ammonium chloride (DMOAP)and trichloro(1H,1H,2H,2H-perfluoro-octyl)silane can be added to thehydrogel before conversion to an aerogel. This type of surfacemodification provides a hydrophobic aerogel that is stable upon exposureto water.

Ordered Colloidal Dispersions within Gels

Colloidal particles having lengths ranging from about 1 nm to 10 μm canbe homogeneously dispersed within the gels. In one embodiment thecolloids can include gold and silver plasmonic nanoparticles such asrods, triangular platelets, and triangular frames; ferromagneticnanoparticles such as ferromagnetic nanoplatelets; quantum dots such asnano-spheres, -cubes, -rods, and the like. The colloidal particles areintroduced into the cellulose before cross-linking occurs.Cross-linking, through interaction with the colloidal surface ligands,binds the colloidal inclusions to the ordered cellulosic network asdescribed previously herein. Any of the disclosed gels can contain oneor more types of colloidal particles.

The disclosed gels have a thickness from about 1 μm to about 10 cm. Inone embodiment the thickness varies from about 10 μm to about 1 cm. Inanother embodiment the thickness varies from about 100 μm to about 10cm. In a further embodiment the thickness varies from about 50 μm toabout 1 cm. In still further embodiment the thickness varies from about1 cm to about 10 cm. In a yet another embodiment the thickness variesfrom about 10 μm to about 100 cm. In a yet still further embodiment thethickness varies from about 500 μm to about 10 cm.

The transmissivity of the disclosed gels relates to the amount ofelectromagnetic radiation that is blocked from passing through the gel.0% transmission results in an opaque material which allows notransmission. 100% transmission results in a material that istransparent to electromagnetic radiation. The disclosed gels can have atransmission of from 0% to 100%. In one embodiment the gels have atransmission of from about 5% to about 15%. In another embodiment thegels have a transmission of from about 25% to about 50%. In a furtherembodiment the gels have a transmission of from about 95% to 100%. In astill further embodiment the gels have a transmission of from about 15%to about 35%. In a yet further embodiment the gels have a transmissionof from about 50% to about 75%. In a yet another embodiment the gelshave a transmission of from about 25% to about 75%.

The disclosed gels and composite materials can have a thermalconductivity of from about 10⁻³ W/(m·K) to about 10 W/(m·K). In anotherembodiment the thermal conductivity is from about 10⁻² W/(m·K) to about10 W/(m·K). In a further embodiment the thermal conductivity is fromabout 10¹ W/(m·K) to about 10 W/(m·K). In a still further embodiment thethermal conductivity is from about 10⁻³ W/(m·K) to about 1 W/(m·K). Inyet further embodiment the thermal conductivity is from about 10⁻²W/(m·K) to about 1 W/(m·K). In yet another embodiment the thermalconductivity is from about 1 W/(m·K) to about 10 W/(m·K).

The emission value of the disclosed gels ranges from about 10⁻² to 0.99.

The disclosed gels and composites can have a bulk modulus of from about1 Pa to about 10⁶ Pa. In one embodiment the modulus is from about 10 Pato about 10⁵ Pa. In another embodiment the modulus is from about 10² Pato about 10⁶ Pa. In a further embodiment the modulus is from about 10³Pa to about 10⁵ Pa. In a still further embodiment the modulus is fromabout 10 Pa to about 10³ Pa. In a yet further embodiment the modulus isfrom about 1 Pa to about 10 Pa. In a yet another embodiment the modulusis from about 10⁴ Pa to about 10⁶ Pa.

Procedures

Cellulosic starting material for the disclosed gels can be derived froma variety of sources. The surface-sulfuricated cellulose particles (FIG.1(a) and FIG. 2) that are suspended in water can be prepared by sulfuricacid-mediated oxidation of the natural cellulose. Thesurface-carboxylated cellulose nanofibers and nanoribbons that aresuspended in water can be prepared by2,2,6,6-tetramethyl-1-piperidinyloxy (TEMPO)-mediated oxidation of thenatural cellulose. The dispersions of these nanomaterials spontaneouslyform a thermodynamically stable cholesteric (for cellulose nanorods) ornematic (for cellulose nanofibers or nanoribbons) liquid crystal phasethat can be transformed into a hydrogel (FIG. 1(b) and FIG. 3(a)) with asimilarly ordered spatial organization of cellulose particles.

Cellulosic starting material for the disclosed aerogels can bebiosynthesized by Acetobacter xylinum utilizing, for example, glucose asa carbon source. Acetobacter xylinum can be cultivated in a glucosemedium for 1-3 weeks under static conditions to produce cellulosepellicles. To remove bacterial cell debris, bacterial cellulose can beboiled in a 1 wt. % NaOH aqueous solution for 2 hours, followed bywashing with water and neutralization with 0.2% acetic acid.

Natural cellulose material obtained in this way can easily bedisintegrated into individual nanocrystals by a controlled TEMPOmediated oxidation. For example, 2 g of bacterial cellulose wassuspended in water (150 mL) containing TEMPO (0.025 g) and NaBr (0.25g). A 1.8 M NaClO solution (4 mL) was added, and the pH of thesuspension was maintained at 10 by adding 0.5 M NaOH. When no moredecrease in pH was observed, the reaction was finished. The pH is thenadjusted to 7 by adding 0.5M HCl. The TEMPO-oxidized products werecellulose nanorods of controlled 4-10 nm diameter and 1000-3000 nmlength, which were then thoroughly washed with water by filtration &stored at 4° C. The aqueous suspension of cellulose nanocrystals abovethe critical concentration (˜0.1-1% wt. %) self-assemble into liquidcrystalline structures-chiral nematic phase, which shows periodichelicoidal structures with a pitch that can be controlled in the range5-70 μm. This structure can strongly reflect electromagnetic radiationof the wavelength comparable with the pitch, which simultaneously servesas a high efficiency low-emissivity structure by tuning the pitch to beappropriate range. The disclosed films can have a pre-designed gradientof cholesteric pitch in helicoidally ordered nanocrystal self-assemblyachieve broadband infrared selective reflection and low-emissivity(compare to the red infrared emission curve taken from solicitation)while being transparent in the visible spectral range & alsotransmitting solar radiation (blue curve) in near-IR range.

The concentration of the liquid crystalline cellulose nanocrystaldispersions were then adjusted to 0.1 wt. % and used in preliminarystudies. The chiral nematic liquid crystalline order of the nanocrystalswas then poured into a mold and the orientation of the helix could bealigned uniformly perpendicular to the film plane using a circularshearing. The polarizing optical micrograph shown in FIG. 4c wasobtained for a sample with the helical axis aligned in the horizontaldirection, providing a side view of the periodic helicoidal structure ofthe chiral nematic liquid crystal. In the design and fabrication of AIRFILMs, this helical axis will be set orthogonal to the plane of the filmand along the normal to the window (FIG. 2), which will be achievedusing the process of circular shearing (see, Park J. H. et al.“Macroscopic control of helix orientation in films dried fromcholesteric liquid-crystalline cellulose nanocrystal suspensions”, ChemPhys Chem. 15, 1477-1484 (2014). The suspension of nanocrystals in thecholesteric phase can be “fixed” using a dilute acid solution (1M HCl),so that the fluid cellulose nanocrystals dispersions transformed intohighly transparent ordered hydrogels.

After cellulose nanomaterial preparation from one or multiple sourcesthe cellulose nanomaterial can be dispersed in water and aligned by thefollowing methods. First, the aqueous suspension of cellulose nanorodsabove the critical concentration (about 0.1-4%) self-assemble into aliquid crystalline phase with nematic or cholesteric ordering. Second,in order to induce uniform alignment across the entire dispersion,linear or circular shearing can be used for nematic or cholestericphases, respectively. Specifically, the dispersion can be confinedbetween glass plates in a mold such that, when a shear stress is appliedfrom the plates in the specified direction (along a line or through arotation), individual nanomaterial directors align to form a singulardirector alignment across the confined dispersion. As an alternate butcomplementary approach to dispersion alignment, the nanocrystals sufferuniform alignment under extrusion from a sufficiently small diameternozzle, syringe, or similar device. With extrusion alignment, noconfining plates are needed such that the aligned dispersion takes theform of a narrow bead, with linear extent much greater thancross-sectional extent, which rests on a supportive substrate or otherstructure. Additionally, the magnetic anisotropy of cellulosenanomaterial's relative permeability can be exploited to cause uniformalignment of nanocellulose. Under sufficiently strong magnetic fields(about 1 T), uniform alignment of nanocrystals is achieved through themagnetic interaction of the induced magnetic dipole moments of thecellulose nanomaterial with the applied magnetic field. (An oscillatingelectric field can be used in a similar manner.)

The aligned cellulose nanomaterial dispersion can be cross-linked toconvert into a hydrogel while preserving its ordering. The extent ofcross-linking of chains of cellulose nanomaterials establishes thedegree to which uniform ordering is preserved in the dispersion. Thatis, for loosely cross-linked cellulose nanomaterials, weak gelationresults. Conversely, strong cross-linking yields firm gelation. Gelationresults from the addition of an acid, a photoacid generator and exposureto light, an alcohol, or another cationic exchange mechanism to theuniformly ordered cellulose nanomaterial dispersion. Alternatively, thecellulose nanorods, inorganic/cellulose nanorods, or polymeric/cellulosenanorods can self-assemble into cholesteric phase by evaporation.

The nanocellulose-based ordered hydrogel was transformed into organogels(FIG. 1(c) and FIG. 3(b)) by shaking the cellulose-nanomaterial hydrogelgently in an organic-solvent-filled bath while replacing the solventregularly in order to prompt the replacement of water in the hydrogel bythe organic solvent. As one example, twice per day for an extent ofthree days ethanol was replaced to facilitate total solvent exchange.

The disclosed cellulose aerogels (FIGS. 1(d) and 3(c)) can be producedfrom the disclosed nano-structured organogel herein above. The resultingcellulose nanocrystal aerogel is a porous material with a skeleton ofabout 0.1-99.9% cellulose nanocrystals and a porosity of about0.1-99.9%. To prevent deformation and crumbling of aerogels during thedrying stage due to surface tension and capillary pressure in theambient atmosphere, supercritical drying, freeze drying, or ambientdrying low-surface tension solvent are used to remove liquid solventfrom cellulose nanocrystal composites while maintaining the disclosedliquid crystalline structure, such as nematic or cholesteric liquidcrystalline ordering.

The disclosed cellulose LC gels (FIG. 1(e) and FIG. 3(d)) can beproduced from an ordered nano-structured organogel, in which an organicsolvent completely miscible with LC is chosen. The organogel is placedin a bath of LC above the boiling point of the organic solvent so thatthe organic solvent is replaced with LC, which functions as the gel'sreplacement solvent.

The disclosed cellulose-templated ordered inorganic and polymeric gelsor films can be produced by mixing cellulose nanomaterials with silicaprecursors or prepolymer and drying the composites in the ambientenvironment. Then the cellulose nanomaterials will be removed either byacidic (for silica aerogel) or basic (for polymeric aerogel) treatmentto form a hydrogel. The hydrogel can be further transferred intoorganogel, aerogel, and LC gel based on the methods described above.

Example 1: Ordered Gels Made from Cellulose Nanorods

Colloidal suspensions of cellulose nanocrystals (CNCs) composed ofcellulose nanorods were prepared by controlled sulfuric acid hydrolysisof cotton fibers, according to the method described by Revol andco-workers (J.-F. Revol, H. Bradford, J. Giasson, R. H. Marchessault, D.G. Gray, Int. J. Biol. Macromol. 14, 170 (1992)). During this process,disordered or paracrystalline regions of cellulose are preferentiallyhydrolyzed, whereas crystalline regions, which have a higher resistanceto acid, remain intact. 7 g of cotton was added to 200 g of 65 wt. %sulfuric acid and stirred at 45° C. in a water bath for up to severalhours until the cellulose had fully hydrolyzed. The mixture wassonicated occasionally, which was found to help degrade the amorphouscellulose regions. The suspensions of cellulose were then centrifuged at9000 rpm for 10 min and re-dispersed in deionized water 6 times toremove the excess sulfuric acid. The resulting precipitate was placedinto a dialysis tubing (MWCO 12000-14000, Thermo Fisher Scientific Inc.)in de-ionized water for three days until the water pH remained constant.The dimensions of CNCs are about 5-10 nm in cross-section and, onaverage, 100-300 nm in length. Then 3 wt. % CNCs solution was cast in amold and evaporated under ambient conditions to obtain an aerogel.

Example 2: Ordered Gels Made from Cellulose Nanofibers or Nanoribbons

Cellulose nanofibers with the dimension of 4.8 nm by several micrometerswere synthesized following the literature (T. Saito, M. Hirota, N.Tamura, S. Kimura, H. Fukuzumi, L. Heux and A. Isogai,Biomacromolecules, 10, 1992 (2009)). Briefly, cellulose based bleachedcoffee filter (1 g) was suspended in 0.05 M sodium phosphate buffer (90mL, pH 6.8) dissolving 16 mg of 2,2,6,6-tetramethylpiperidine-1-oxylradical (TEMPO) and 1.13 g of 80% sodium chlorite in an airtight flask.The 2 M sodium hypochlorite solution (0.5 mL, 1.0 mmol) was diluted to0.1 M with the same 0.05 M buffer used as the oxidation medium and wasadded at one step to the flask. The flask was immediately stoppered, andthe suspension was stirred at 500 rpm and 60° C. for 96 hours. Aftercooling the suspension to room temperature, the TEMPO-oxidized cellulosewas thoroughly washed with water by filtration. Then 0.1-1.0 vol. % CNFsaqueous dispersion was poured into a mold, aligned by a shear force, andseveral drops of 1 M HCl solution was added to form a hydrogel after 2hours. The hydrogel was immersed into ethanol for 2 days for solventexchange to form an organogel. To form a liquid-crystal gel, theorganogel was immersed in a bath of liquid crystal4-cyano-4′-pentylbiphenyl at 90° C. for 12 hours. Subsequent cooling toroom temperature after solvent exchange caused the LC to enter itsexpected nematic phase. The aerogel was formed by critical point dryingof the organogel.

Example 3: Ordered Gels Made from Cellulose-Nanorods-Templated Silica

CNCs were synthesized according the method in Example 1. Then 5 mL of 3wt. % CNCs dispersion was mixed with 10-75 μL of silica precursortetramethyl orthosilicate and stirred for 1 hour. Then the compositeswere cast into a Petri dish and dried over 1-2 days. The CNCs were thenremoved by pyrolysis method (e.g. 540° C. for 20 hours) or keeping in16% sodium hydroxide solution for 16 hours. The silica hydrogel wasformed and can be further transferred into organogel by solvent exchangeand aerogel by critical point drying or ambient drying.

Example 4: Ordered Gels Made from Cellulose-Nanorods-Templated Polymer

CNCs were synthesized according the method in Example 1. Then 5 mL of 3wt. % CNCs dispersion was mixed with 10-75 mg of water-soluble preformedpolymer and stirred for 10 min. Then the composites were casted into aPetri dish and dried over 1-2 days. The film was then cured atpolymerization temperature for 24 hours. The CNCs were then removed by16% sodium hydroxide solution for 16 hours. The polymer hydrogel wasformed and can be further transferred into organogel by solvent exchangeand aerogel by critical point drying or ambient drying.

The light scattered upon passing through aerogels can produce a hazyappearance, which can result in the reduction of contrast of objectsviewed through the film. Haze measurements can be performed using ahazemeter or spectrometer. FIG. 5 describes an apparatus developed andadapted to measure the quality of the disclosed aerogels. The apparatusin FIG. 5 has the advantage of conducting haze measurements that alsoprovides diagnostic data on the origins of the haze.

The typical setup includes an integrating sphere where the measured filmis placed against the sphere entrance port. The surface of the interiorof the integrating sphere is highly reflecting throughout the visiblewavelengths obtained from light sources. The light entering theintegrating sphere is reflected from the surface towards the testedfilm. Then the light transmitted through the film is focused anddirected to a photodetector. A photodetector in the spectrophotometersetup is computer driven and values for transmission and haze can beautomatically calculated. The haze can be determined ashaze=100×(T_(d)/T_(t)), where T_(t) is a total transmittance dependingon intensity of incident light & total light transmitted by the film &T_(d) is diffuse transmittance depending on light scattered by ameasuring setup & the film.

Measurement of Sound Proofing Characteristics

Sound proofing characterization can be conducted by imaging ofnanoparticles using TEM and SEM. FIG. 6 outlines a procedure for probingof the pore size, surface area, and structural properties of cholestericcellulose-based aerogels under different preparation conditions. Theordering of nanocrystals on large scales will be probed using 3D imagingtechniques such as Fluorescence Confocal Polarizing Microscopy (FCPM),Coherent Anti-Stokes Raman Scattering Polarizing Microscopy (CARS-PM),and Three-Photon Excitation Fluorescence Polarizing Microscopy(3PEF-PM). in addition, the monitoring of the disclosed liquid crystaland aerogel uniformity in lateral directions can be accomplished byusing conventional dark-field, bright-field, and polarizing opticalmicroscopy. Visible- and infrared-range spectroscopy can also beutilized. The other properties of the disclosed aerogels includemechanical properties (both as-prepared aerogels and encapsulated,ready-to-install films), as well as soundproofing and condensationresistance of the films installed on single-pane windows.

Measurement of Aerogel Thermophysical Properties

Prior to the application of films onto a window the thermophysicalproperties (for example, thermal conductivity, heat capacity) arecharacterized by the optical pump-and-probe method in order to determinethe type of film that is suitable for the specific application. Thisprocedure uses a femtosecond laser to construct a high temporalresolution temperature measurement system. FIG. 7 shows the opticalpathway of a typical pump-and-probe measurement system. In the opticalpump and probe method, sub-picosecond (ps) time resolution is madepossible by splitting the ultrafast sub-ps laser pulse output into anintense heating pulse, i.e., a “pump” beam, and a weaker “probe” beam,and controlling the optical path length difference of the pump and probebeam through a mechanical delay stage. The decay of the temperature riseis measured by the reflected energy of the probe pulse series. Thethermal conductivity can then be deduced by fitting the temperaturedecay curves.

Condensation Resistance

After determining the thermal conductivity of a disclosed aerogel, filmscan be applied atop of a window to determine the thermal insulation andcondensation resistance performance, following the current standards ofASTM C1363-11 and ASTM C1199-14. This test method establishes principlesfor design of a hot box apparatus & requirements for the determinationof the steady-state thermal performance of windows when exposed tocontrolled laboratory conditions. The window thermal insulation andcondensation performance is represented by the overall heat transfercoefficient, U. FIG. 8 discloses the components of a hot box apparatus:a metering box (simulating interior temperature) on one side of thewindow specimen; a controlled guard box surrounding the metering box; aclimate chamber box (simulating exterior temperature) on the other side;and the specimen frame providing specimen support & insulation.

The walls of the hot box are insulated panels of plywood adhered toeither side of a solid layer of XPS insulation. The space conditioningsystem used in the meter box employs hydronic cooling and electricresistance heating. The meter box cooling is measured using highprecision thermocouples in combination with a precision flow meter toaccurately quantify the heat removed from the meter box. Heat is addedinto the meter box via PM-controlled electric heaters. Precisionresistor circuits are employed to measure the heat added into the meterbox. A constant and precise temperature can be maintained and the totalheat addition/removal can be measured. The hot box employs an insulatedguard box surrounds the meter box and a hydronic guard loop is installedover the outside surface of the meter box. The guard box minimizes theinfluence of temperature changes in the lab. The liquid guard loopfurther ensures the outside surface of the meter box remains at aconstant temperature. An insulated baffle separates the air space fromthe mixing chamber of the meter box. The baffle panels are constructedusing thermal insulation material. For a 1 m×1 m test window sample, atleast 25 calibrated precision thermocouples are used to measuretemperatures on the baffle surface, 25 corresponding points in the airstream and at least 25 points on the interior surface of the test windowspecimen. Air drawn through the meter box baffle space at velocitiesrepresentative of convection in real world conditions. DC powered axialdraw-through circulation fans, at the top and bottom of the baffle, areused to ensure smooth flow along the surface of the wall sample in thedirection that convection would occur.

The climate box has the same dimensions and construction as the guardbox. The climate side air baffles are constructed using the samematerials and methods as the air baffles in the meter box. Heat is addedto/removed from the climate box via four fan coils which are connectedto a liquid chiller and a hydronic heater. Electric resistance heaterspermits fine tuning of the temperatures and ensures that temperaturesremain close to the set point for the duration of the test. The climatebox has the capacity to run a range of realistic outdoor temperatures,from −30° C. to 60° C. This enables the tested window assembly to remainundisturbed when tested from cold to hot climate conditions. Overallheat transfer coefficient is:

$U = \frac{Q}{A \cdot \left( {T_{meter} - T_{climate}} \right)}$

where Q is the time rate of net heat flow through the meter box opening,W; A is meter box opening area, m²; T_(meter) and T_(climate) aretemperatures of meter box and climate box, respectively.

Cellulose-Polysiloxane Hybrid Aerogels

Further disclosed are transparent cellulose-polysiloxane hybridhydrogels, organogels and aerogels and a process for preparing thedisclosed transparent cellulose-polysiloxane hybrid transparentcellulose-polysiloxane hybrid hydrogels, organogels and aerogels.Disclosed is a cellulose-polysiloxane hybrid aerogel, comprising:

a) a cellulosic matrix; and

b) a polysiloxane surface network

Without wishing to be limited by theory, these characteristics areachieved by strictly controlling the dimensions of nanofibers and thehomogeneous gel skeleton networks that they form, which can be tuned toform orientationally ordered liquid crystal (LC) states. In the gelfabrication process, an optimized acid/base catalyzed sol-gel reactionin a surfactant-based solution is used to form apolymethylsilsesquioxane (PMSQ) surface network.

Cellulose nanofibers having a uniform diameter are first surfacefunctionalized. This functionalization can employ small chargedmolecules or polymer grafting resulting in increased cellulose nanofiberstability. Subsequently, these nanofibers are crosslinked with PMSQfibers. In addition to their high optical transparency, super thermalinsulation, flexibility and mechanical robustness, the disclosed hybridaerogels can be made optically isotropic or anisotropic, depending onthe intended use by the formulator. In the case of anisotropic aerogels,they can be fabricated starting from the LC states of colloidaldispersions of nanofibers. The resulting compositions can have practicalapplications as they result can result in devices having opticalpolarization, thereby the ability to control visible light polarizationwhile providing simultaneous thermal insulation.

Disclosed herein is a process for preparing polymethylsilsesquioxane(PMSQ) network cellulosic aerogels, comprising:

-   -   a) contacting an aqueous dispersion of cellulose with a        oxidizing system that oxidizes the C6 hydroxyl units of        cellulose to carboxylate units to form an aqueous solution of        oxidized cellulose nanofibers;    -   b) reacting the oxidized cellulose nanofibers with a surface        modifying agent to form an aqueous solution of surface modified        cellulose nanofibers;    -   c) contacting the surface modified cellulose nanofibers with        polymethylsilsesquioxane (PMSQ) to form an aqueous polysiloxane        precursor;    -   d) hydrolyzing the polysiloxane precursor in the presence of an        acid catalyst to form a PMSQ network cellulosic hydrogel;    -   e) exchanging the water contained in the hydrogel with a        volatile solvent to form an organogel; and    -   f) removing the volatile solvent to form an aerogel.

In one embodiment the oxidizing system comprises:

-   -   a) an admixture of (2,2,6,6-Tetramethylpiperidin-1-yl)oxyl        (TEMPO) and NaClO.

Modifying Agents

In one embodiment the surface modifying agent is chosen from C₁-C₆linear or branched, saturated or unsaturated alkylamine, low molecularweight compounds comprising a cationic moiety, oligomers or polymers.

The C₁-C₆ linear or branched, saturated or unsaturated alkylamines reactwith the cellulose carboxyl units under the conditions of the presentprocess to form a carboxylate-quaternary ammonium complex, for example,as depicted in FIG. 9A. One example of this embodiment comprises the useof allylamine as the modifying agent.

Another embodiment comprises the use of an oligomer or polymer as themodifying agent. In one iteration m-PEG-amines having an averagemolecular weight of from about 2000 to about 10,000 daltons are used tomodify the surface of the oxidized cellulose nanofibers. In onenon-limiting example of this iteration the modifying agent is an m-PEGamine having an average molecular weight of 5000 daltons. For example,the m-PEG amine depicted in FIG. 9C wherein the index n is approximately112. Any oligomer or polymer that can covalently bond to the surfacecarboxylates of the oxidized nanofibers can be used to modify thecellulosic surface.

A further embodiment comprises a modifying unit that is a low molecularweight compound comprising a cationic moiety. The molecular weight ofcompounds of this type are less than about 400 g/mol. A non-limitingexample of this embodiment is depicted in FIG. 9B wherein the use of acarbamoylcholine salt is the modifying agent. The salt can be chlorine,bromine and the like.

General Procedure

In order for spontaneous nematic ordering of the nanofibers to occur,the nanocellulose concentration must be above the criticalconcentration. This behavior provides a unique opportunity for theformulator to impart LC ordering at low TOCN volume fractions whichprovides a means for obtaining the disclosed optical anisotropy andother properties.

Raw cellulosic material obtained from natural sources is used to formthe disclosed hydrogels, organogels and aerogels. For example, cotton,grains, paper products made from natural sources and the like. Thecellulosic material is first oxidized at the C6 saccharide carbonsthereby oxidizing the —CH₂OH moieties to carboxylate moieties, —COOH.

The oxidized cellulosic material is then treated with a modifying agent,which allows the cellulose nanofibers in the hydrogel to remain alignedand non-reactive to the subsequent treatment with the networking agent.Next, following treatment with the modifying agent, the nanofibers aretreated with methyltrimethoxysilane (networking agent) which ishydrolyzed under acidic conditions to form a polysiloxane network overthe hydrogel. Gelation results in a highly-transparent monolithichydrogel of functionalized TOCNs, cross-linked by an isotropic,bicontinuous polysiloxane nanofibrous network.

The water is removed from the hydrogel by exchange with a volatileorganic solvent to form the corresponding organogel. The resultingorganogel exhibits both an isotropic and liquid-crystalline arrangement,which can be controlled by regulating the surface-modified TOCNconcentration. These orientationally ordered self-assembled structuresare locked in place by the formation of the polysiloxane network. Thedisclosed process preserves the small and uniform cross-sections ofindividual fibers and their network and, consequently, assures low lightscattering.

The corresponding aerogel is formed by drying of the organogel, whichcan be shaped to the needs of the formulator. In addition to mechanicalflexibility and robustness, many practical aerogel applications canrequire a high degree of hydrophobicity (for example, to assure thatthese aerogels are stable under ambient conditions and in humidenvironments).

Example 5: PMSQ Network Cellulosic Aerogels

The disclosed cellulose nanofibers are produced through the oxidation ofnative cellulose by selectively modifying the C6 primary hydroxyl groupson the surface of native cellulose to carboxylate groups catalyzed by2,2,6,6-tetramethylpiperidine-1-oxyl radical (TEMPO) under mild pHaqueous conditions, known as TEMPO-oxidized cellulose nanofibers(TOCNs). The nanofibers with a diameter of 4.8 nm and micrometer-scalelengths are stabilized in a basic solution by the Coulombic repulsion oftheir anionic carboxylate moieties, which overpower their tendency toform hydrogen bonds. As a result, the aqueous TOCN dispersions arehighly transparent. To eliminate the strong light scattering originatingfrom bundling and clustering of TOCNs which are uncontrollablycrosslinked by direct hydrogel bonds, as often observed in polymericfibrous aerogels, we instead cross-link TOCNs with polysiloxane. Thistechnique precludes the direct contacts between TOCNs and therebygenerates a uniform nanofibrous network that exhibits small scatteringcross-sectional areas. Hydrolysis of the polysiloxane precursor isacid-catalyzed. However, even under mildly acidic conditions and diluteconcentrations, TOCNs tend to form a gel-like phase due to the hydrogenbonding between carboxylic acid functional groups. To stably disperseTOCNs in polysiloxane precursor solutions, we implement various TOCNsurface functionalization schemes, as illustrated in FIGS. 9A-9C.

The TEMPO-mediated oxidation of cellulose produces a large density ofcarboxylic groups (˜0.8 mmol/g) on the surface of nanofibrillatedcellulose that is available for surface modification. This provides ameans for altering the physical adsorption properties of the cellulosenanoparticles by covalently bonding either low molecular weight cationicmolecules or polymeric chains to the surface thereby resulting instabilized TOCNs by either electrostatic repulsion or steric hindrance.

In one embodiment as depicted in FIG. 9A this modification is affectedby physisorption of one or more polyelectrolytic monomers, in thisexample allylamine, to the anionic carboxylate groups of the oxidizedcellulose. This process does not significantly affect thecross-sectional diameter of the TOCN's because of the size of the lowmolecular weight of the cationic small molecule.

In another embodiment as depicted in FIG. 9B the surfacefunctionalization can be accomplished by reaction of the carboxyl groupswith a cationic-amine comprising adduct. FIG. 9B depicts the reaction of2-(carbamoyloxy)-N,N,N-trimethylethanaminium (choline carbamate) withthe TOCN's. This reaction introduces another form of cationic chargeelectrostatic repulsion.

In a further embodiment as depicted in FIG. 9C the surface of the TOCN'sare modified by reaction with a polymeric material, in this example amethoxy polyethylene glycol amine (mPEG-amine). Grafting of a polymer ofthis type provides a means for improving colloidal-TOCN stabilization.

The functionalization of the TOCN's produces cellulosic matrices thatare stable to treatment with polysiloxane in the subsequent step of thedisclosed process.

FIG. 10 depicts the process in general. Oxidized and surface modifiedcellulose nanofibers as an aqueous suspension are represented by thelong aligned fibers and the differently shaded dots represent watermolecules and molecules of PMSQ (far left figure). The center figurerepresents Steps (c) and (d) of the process above wherein the nanofibersare first contacted with PMSQ then the PMSQ is hydrolyzed to form a PMSQnetwork cellulosic hydrogel. The resultant of Steps (e) and (f) isdepicted in the figure on the far right, the resulting aerogel. Theresulting transparent surface-modified TOCNs' aqueous colloidaldispersions can exhibit LC ordering, depending upon the volume fractionof the nanofibers in the colloidal dispersion. In addition, thesesolutions can exhibit birefringence when they are observed between crosspolarizers as depicted in FIG. 10.

Preparation of TEMPO-Oxidized Cellulose Nanofiber.

TEMPO having the dimensions of 4.8 nm by several micrometers wereprepared using bleached wood cellulose as follows. Wood-cellulose-basedbleached coffee filter (1 g) was suspended in 0.05 M sodium phosphatebuffer (90 mL, pH 6.8) by dissolving 16 mg of TEMPO and 1.13 g of 80%sodium chlorite in a reaction flask. Then 455 μL of NaClO solution (13%active chlorine) was diluted ten times with the same 0.05 M sodiumphosphate buffer and was added in one portion to the reaction flask. Theflask was immediately stoppered, and the suspension was stirred at 500rpm at 60° C. for 120 hours. After cooling the suspension to roomtemperature, the TEMPO-oxidized cellulose fibrils were thoroughly washedwith water by centrifugation at 8700 rpm for 30 min. TEMPO-pretreatedcellulose fibrils were then diluted at 0.25 wt. %, mechanically blendedat 28,000 rpm by a food processor, homogenized using a tip sonifier andfiltered using a membrane filter with a pore-size of 11 μm. Theresulting transparent solution was then concentrated by a rotaryevaporator at 60° C.

Surface Modification of TOCN 1. Cationic Surface Physisorption

The surface of the TEMPO-oxidized cellulose nanofibers were thenfunctionalized by physical adsorption of allylamine onto the nanofibers.500 mg of 0.2 wt. % nanofiber aqueous solution was diluted by 2 mL ofdistilled and deionized water and combined with 10 mg of allylamine. Themixture was stirred overnight and dialyzed for 2 days in a deionizedwater bath across a cellulose acetate membrane with a cutoff molecularweight of 12,000-14,000 g/mol to obtain the desired allylamine-TOCNs.

2. Charged Small Molecule Modification

An aqueous TOCN dispersion (500 mg of 0.38 wt. %) was diluted with 2 mLof deionized water followed by the addition of 24 mg of1-[bis(dimethylamino)methylene]-1H-1,2,3-triazolo[4,5-b]pyridinium3-oxid hexafluorophosphate (HATU), 20 μL of N,N-diisopropyl-ethylamine(DIPEA) and 50 mg carbamoylcholine chloride and 40 μL dimethylformamide(DMF). The mixture was stirred for 2 days and dialyzed for another 2days affording the desired dispersed surface modified nanofibers.

3. Oligomer/Polymer Modification

An aqueous TOCN dispersion (500 mg of 0.2 wt. %) was diluted with 2 mLof DI water and followed by mixing with 28 mg of HATU, 20 μL of DIPEA,18 mg mPEG-amine (MW=5000) and 40 μL DMF. The mixture was stirred for 2days and then dialyzed for another 2 days to finally obtain mPEG-TOCNs.All of the functionalized TOCN dispersions were concentrated by a rotaryevaporator to the desired concentration.

Preparation of PMSQ Network Cellulosic Hydrogels

The disclosed PMSQ network cellulosic hydrogels were fabricated bycross-linking functional TOCNs with polysiloxane. For example and ingeneral, cetyltrimethylammonium bromide (0.4 g) (CTAB) and 3.0 g of ureawere dissolved in 8 mL of deionized (DI) water with sonication until thesol became homogeneous. To this solution is added 2 mL of afunctionalized surface modified TOCN at differing concentrations, 1-5 mLof methyltrimethoxysilane (MTMS) and 0.01 mmol acetic acid undervigorous stirring. After stirring each sample for 30 minutes at roomtemperature, the sol was degassed in a vacuum oven and then transferredinto a polystyrene petri dish with a diameter of 5 cm, sealed forgelation and allowed to age for 3 days in a 60° C. furnace to form thedesired hydrogels.

Preparation of PMSQ Network Cellulosic Aerogels

The hydrogels formed above were taken from the petri dish and immersedin DI water for 24 hours to remove the urea and residual CTAB. This wasfollowed by solvent exchange with isopropanol, which was replaced every12 hours, at 60° C. for 2 days. Finally, CO₂ supercritical drying at 38°C. under 8.6 MPa was conducted to obtain dried aerogel samples in acritical point dryer. This provided aerogels having bulk densitiesranging from 30-200 mg/cm³ depending upon the amount of MTMS added tothe functionalized surface modified cellulose in the above step. In oneembodiment an aerogel having a density of 69 mg/cm³ promotes optimaloptical transmission and mechanical flexibility. In one iteration of thedisclosed process no stress is introduced to TOCN-PMSQ aerogel duringprocessing.

FIG. 11 is a photograph showing the optical transparency of a hydrogelformed from the disclosed process. This hydrogel is a highly-transparentmonolithic hydrogel cross-linked by an isotropic, bicontinuouspolysiloxane nanofibrous network as described herein. The hydrogel iscontained within the outlined dotted area. FIG. 12 is a photographshowing the optical transparency of an organogel formed from thedisclosed process. The organogel is contained within the outlined dottedarea. FIG. 13 is photograph showing the optical transparency of anaerogel formed from the disclosed process wherein the surface modifyingagent is allylamine. The aerogel is contained within the circle. FIG. 14is a photograph of an aerogel formed by the disclosed process whereinthe surface modifying agent is an m-PEG-amine having an averagemolecular weight of 5000 daltons. The circular aerogel is positioned ontop of a copy of text. AS can be seen in the photograph the aerogel istransparent in that neither the color nor the text is distorted. FIG. 15is a photograph of an aerogel formed by the disclosed process whereinthe surface modifying agent is carbamoylcholine chloride. The circularaerogel is positioned on top of a copy of text. The aerogel istransparent in that neither the color nor the text is distorted.

As depicted in FIG. 16 the carbamoylcholine chloride-capped TOCN-PMSQaerogels exhibit hydrophobic surface characteristics with a typicalcontact angle of 148°, largely due to the presence of hydrophobic methylgroups on the polysiloxane fibers within the nanostructured aerogels. Anadvantage of the disclosed process is that there is no need forpost-synthetic hydrophobization treatment when the disclosed gels areused for hydrophobic applications.

The disclosed aerogels were analyzed for both their optical and electronimaging and spectra characteristics. For both polarized and unpolarizedbrightfield optical microscopic imaging, an Olympus BX-51 polarizingoptical microscope was equipped with 10×, 20×, and 50× air objectiveswith a numerical aperture NA=0.3-0.9 and a CCD camera Spot 14.2ColorMosaic (Diagnostic Instruments, Inc.). Transmission spectra werestudied using a spectrometer USB2000-FLG (Ocean Optics) mounted on themicroscope. For light transmittance and haze measurements of aerogels, aUV-VIS-NIR spectrometer, ranging from 190 nm to 3200 nm, (UV-3101pc,from Shimadzu) equipped with a Lab Sphere brand integrating sphereattachment was employed. Haze is defined as the ratio of diffusetransmission to total transmission, where diffuse transmission isdefined as transmitted light varying by greater than or equal to a 5°separation from the direction of incident light. Infrared transmissionspectra from wavenumbers 400 cm⁻¹ to 4000 cm⁻¹ (wavelengths 2.5 μm-25μm) were recorded on a Fourier-transform infrared spectroscopy (FTIR)spectrometer (Nicolet AVATAR 370 DTGS from Thermo) equipped with an aintegrating sphere (NIR IntegratIR, from Pike). Photographs of sampleswere taken using a digital camera. IR thermographs were obtained by anIR camera (T630sc, from FLIR). TEM images were obtained using a CM100microscope (from FEI Philips) at 80 kV. The TOCN samples were negativelystained with phosphotungstic acid to increase imaging contrast: 2 μL ofthe sample is dropcasted on the formvar coated copper grid, allowed tosettle for drying and then dipped into the stain solution (aqueous 2 wt.% phosphotungstic acid). The porous morphology of TOCN-PMSQ wascharacterized using an SEM using a Hitachi Su3500 and Carl Zeiss EVO MA10 system. For this, freshly cut surfaces of the TOCN-PMSQ aerogels weresputtered with a thin layer of gold and observed under SEM at a lowvoltage of 5 kV (as optimized to avoid the distortion of the aerogelsamples).

FIGS. 17A-17C are transmission electron microscopy (TEM) micrographs ofthe disclosed aerogels at various magnifications. FIG. 17A shows thatthe colloidal dispersions consist of mostly individualized TOCNs, eachof diameter D_(c)≈5 nm and length L_(c)≈1-2 μm. FIGS. 17B and 17C arescanning electron microscopy (SEM) that depict the well-defined anduniform-diameter 10-15 nm nanofibers that are formed by polysiloxanetreatment and individually dispersed TOCNs fibers within the aerogels aswell as a narrow pore-size distribution of their resulting porousnetwork. The depicted aerogel samples exhibit 3D bicontinuousnetwork-like structures, in which both the smooth gel skeletons and thepores are interconnected without aggregation or clustering. The exampledepicted in FIGS. 17A-17C have a bulk density ρ_(b) is calculated to be69 mg/cm³ by weight/volume ratio of the sample. The porosity, defined as=(1-ρ_(b)/ρ_(s))×100%, is then determined to be ε≈94.9%, where ρ_(s) isthe skeletal density taken to be 1.35 g/cm³. The average pore size forthis particular example is calculated to be approximately 100 nm,consistent with the value observed directly from the SEM images. Themesoscale morphology of the 2.0-mm thick QA-capped TOCN-PMSQ compositeaerogel with ultrathin fibers and uniform pore size distribution yieldshydrogels and organogels with very high light transmission greater than90% and aerogels with visible transmission close to 90% at 600 nm asdepicted in FIG. 18.

The exampled aerogel's haze coefficient, defined as the ratio of diffusetransmittance and total light transmittance, was determined to equal to8.4% FIG. 19 and is characterized following the ASTM D1003 standardusing an integrating sphere setup when integrated across the visiblerange (390-700 nm) The PMSQ matrix causes TOCN-PMSQ aerogels to exhibitstrong absorption at a wavelength of 6-20 μm, which is mainly due to theSi—O bonds FIG. 20. This provides the formulator the opportunity toseparately control transmission of visible and infrared light, inembodiments wherein control of solar gain and emissivity are important,i.e., in smart-window applications.

The disclosed aerogels were analyzed for their thermal, mechanical, anddurability characteristics. The thermal conductivity is measured bymeasuring both the heat capacity and thermal diffusivity of the aerogelsamples. The heat capacity of aerogel is measured by differentialscanning calorimetry (DSC 204 F1 Phoenix, Netzsch). The thermaldiffusivity of aerogel is characterized by a laser flash apparatus (LFA457, Netzsch). Briefly, an optical source instantaneously heats one sideof the material and the temperature increment on the other side of thematerial is recorded by infrared thermography for facile, noninvasivetemperature sensing. To prevent the direct heating of the detector bylaser light, the top and the bottom of the aerogel were covered withhighly conductive carbon tape to prevent the laser from penetratingthrough the sample. The thermal conductivity of the aerogel can becalculated by subtracting the contribution of carbon tape from theeffective thermal conductivity of the sandwich structure, which wasdetermined by performing measurements for samples of differentthickness. The Instron 5965 material-test system was used to probe themechanical properties and determine stress-strain relationships. Themechanical properties shown in FIG. 4f were measured with TOCN-PMSQaerogel samples with 0.25 vol. % QA-capped TOCN cut into rectangularstrips of 20 mm×6 mm×1 mm. Aerogel durability testing was performedunder a 500 Watt mercury lamp (Sun System 5, from Sunlight Supply Inc.)and in a Tenney environmental test chambers held at 80° C. and 80%relative humidity for 24 hours.

FIG. 21 shows the measured thermal conductivity of an TOCN-PMSQ aerogelversus sample porosity. FIG. 22 depicts the comparison of thermalconductivity between an aerogel formed from carbamoylcholine chloridemodified nanocellulose (quaternary-amine) and an allylamine modifiedaerogel. FIG. 23 depicts the compression stress-strain relation for aTOCN-PMSQ aerogel with 0.06 wt. % of TOCN.

What is claimed is:
 1. A liquid crystal gel comprising: a) a cellulosicmatrix; and b) one or more solvents.
 2. The gel according to claim 1,wherein the one or more solvents are nematic liquid crystals.
 3. The gelaccording to claim 1, wherein the solvents are chosen from1-(trans-4-hexylcyclohexyl)-4-isothiocyanatobenzene;4′-(hexyloxy)-4-biphenylcarbonitrile;4′-(octyloxy)-4-biphenyl-carbonitrile;4′-(pentyloxy)-4-biphenylcarbonitrile; 4′-heptyl-4-biphenylcarbonitrile;4′-hexyl-4-biphenylcarbonitrile; 4′-octyl-4-biphenylcarbonitrile;4′-pentyl-4-biphenylcarbonitrile; 4,4′-azoxyanisole;4-isothiocyanatophenyl 4-pentyl-bicyclo[2.2.2]octane-1-carboxylate;4-(trans-4-pentylcyclohexyl)benzonitrile; 4-methoxycinnamic acid;N-(4-ethoxybenzylidene)-4-butylaniline; orN-(4-methoxy-benzylidene)-4-butylaniline.
 4. A hydrogel, comprising oneor more cellulose nanomaterials wherein the hydrogel has a thermalconductivity of from about 10⁻³ W/(m·K) to about 10 W/(m·K).
 5. Thehydrogel according to claim 4 wherein the hydrogel has a transmissivityof electromagnetic radiation of less than about 100%.
 6. The hydrogelaccording claim 4, wherein the hydrogel has a bulk modulus of from about1 Pa to about 10⁶ Pa.
 7. The hydrogel according claim 4, wherein thehydrogel comprises cellulosic nanorods.